The Theme of the Book

Millennial-scale (1000-year) climate variability has driven large changes of vegetation and fauna at almost all of the Long-Term Ecological Research (LTER) sites. Decadal climate variability at some sites has seen dramatic changes in fish catches and has altered tree species composition. During the first two decades of study, LTER sites have been affected by two super El Niño events and several more "normal" El Niños and La Niñas. Major droughts have affected species diversity and killed some trees. Severe storms and floods have damaged stream restoration structures. Coastal sites have measured a rise in sea level. Antarctic sites have documented the decrease of some penguin populations and a rise in other populations as a result of climatic warming over 50 or more years. Climate variability has constantly been on investigators' minds. It is little wonder that ecologists clearly recognize climate as a driver of biotic systems. Parmesan and her coworkers describe how climate affects individual fitness, population dynamics, and the distribution and abundance of species, as well as ecosystem structure and function (Parmesan et al. 2000). They relate how regional variation in climatic regimes creates selective pressures for the evolution of locally adapted physiologies, and morphological and behavioral adaptations. They quote the curious fact that climate even determines gender in some species. Map turtles (Graptemys) produce only males if the incubation temperatures are below 28°C and only females if the incubation temperatures are above 30°C (Bull and Vogt 1979). The implications of a steep warming trend for this species are dire! The role of climate as a driver of ecosystems has important practical implications for ecology. For example, Swetnam and Betan-court (1998) make clear that regional climate signals existing in ecosystems must be extracted before variations in ecosystem components can be attributed to other causes.

The theme of this book is how ecosystems respond to climate variability. This theme is examined at a variety of LTER sites and over a variety of timescales. The subject matter of the book is focused on a series of questions that are outlined here. The theme of climate variability and ecosystem response is inherently deterministic and implicitly carries with it the notion of climatic cause and ecosystem result. The analyses in this volume will amply demonstrate that this is a valid and fruitful working assumption. However, we acknowledge that this approach is limited in several senses. First, we recognize that, although in many instances climate may be recognized as the prime ecosystem driver, it is becoming increasingly clear that many ecosystem functions directly or indirectly affect the climate (e.g., Hayden 1998a). Second, there are many factors, both biotic and abiotic, that affect ecosystems besides climate. Third, many internal operations of ecosystems lead to ecosystem response and change. Fourth, many aspects of climate variability and ecosystem response have important implications for human systems. Human activities can sometimes overwhelm or strongly modify climatic influences. The change from grassland to shrubland over the last 150 years at the Jornada and Sevilleta LTER sites is an interesting example (chapters 17 and 15). It is impossible for us to deal with all these aspects, and so some degree of focus is necessary. That focus is provided by the more "simple" climate variability and ecosystem response approach. We also concentrate, for the most part, on results of research conducted at LTER sites. We are well aware that many other researchers and groups are addressing the issue of climate variability and ecosystem response within other contexts.

Despite these caveats, we think it is legitimate to treat climate variability as a prime driver of ecosystem responses. In this volume we also tend to approach climate in isolation from other factors. Climate differs from other ecosystem drivers: It has a certain regularity, expectedness, and predictability. Even in the areas of un certainty, it is often possible to put outer bounds on the kinds and sizes of variability that might be expected. This cannot be said with so great a confidence for many biotic factors. Directional evolutionary trends in some cases, and complete extinction in other cases, make the biotic world a very surprising one. When one adds such anthropogenic factors as land-use change, genetic engineering, and the development of new technologies, the uncertainties mount ever higher. Our approach in dealing with what we know about climate variability and ecosystem response is simple, but it contains the possibility of developing new knowledge.